Precursor seismicity, foreshocks and aftershocks of the Uttarkashi earthquake of October 20, 1991 at Garhwal Himalaya
The Uttarkashi earthquake (m(b) = 6.6) of October 20, 1991 occurred in a complicated geological/tectonic setting at the Main Central Thrust (MCT) zone of the Himalaya. The main shock was preceded by a precursory earthquake activity, which continued for about eight years, and then there was a quiescent period for about three years. Three foreshocks, M > 4.0, occurred 48 h before the main shock, two aftershocks, M > 4.0, occurred within 12 h of the main shock which was then followed by another two aftershocks, M > 4.0, after about seven days, on October 27, 1991. The largest aftershock, M 5.2, occurred within 12 h. The aftershocks, magnitude less than 4.0, continued for about two months, and it decayed at a rate t(-0.87). About 1000 earthquakes, magnitude less than 1.0 and above, were recorded by a closely spaced five-station temporary microearthquake (MEQ) network. Out of these, 124 aftershocks which occurred inside the network are located by Homogeneous Station Method. Spatial variation of the aftershocks with time was observed. Fault-plane solutions of the main shock and aftershocks indicate dominantly thrust faulting. Based on the hypocentral data, depth estimate of the plane of detachment is made, and a fault-interaction model is presented. The model shows that the main shock occurred at the juncture of MCT zone, plane of detachment and the basement thrust. The north-dipping Uttarkashi fault, a parallel fault at the MCT zone, has been activated by the tectonic stress generated due to relative movement of the Tibetan plate over the Indian plate.